1. Field of the Invention
The invention pertains to a hydrodynamic clutch device of the type having a clutch housing connected to a drive; a pump wheel connected to the clutch housing, the pump wheel having a hub; a turbine wheel connected to a takeoff, the turbine wheel and the pump wheel forming a hydrodynamic circuit; a bridging clutch having a piston and at least one friction surface which can be acted on by the piston to connect the drive to the takeoff independently of the hydrodynamic circuit, the piston having one surface facing the friction surface; a pressure space for exerting fluid pressure on the piston; and a pressure circuit including a first pressure medium line to supply the hydrodynamic circuit with clutch fluid and a second pressure medium line to supply the pressure space with clutch fluid.
2. Description of the Related Art
U.S. Pat. No. 5,575,363 describes a hydrodynamic clutch device designed as a hydrodynamic torque converter, the clutch housing of which is attached for rotation in common in the conventional manner to a drive, such as an internal combustion engine. The clutch device comprises a pump wheel, which works together with a turbine wheel and a stator to form a hydrodynamic circuit. Whereas the turbine wheel is connected nonrotatably to a takeoff, such as a gearbox input shaft, the stator is mounted by way of a freewheel on a support shaft, which is provided radially between a pump wheel hub and the gearbox input shaft. In addition, the hydrodynamic clutch device also has a bridging clutch with a piston, which is connected nonrotatably but with freedom of axial movement to the clutch housing.
The hydrodynamic clutch device is designed as a two-line system, as a result of which the following pressure and flow relationships are created:
A first pressure medium line for supplying the hydrodynamic circuit is formed by thrust washers, one of which is located on each side of the freewheel of the stator, and each of which is provided with flow channels for clutch fluid. This first line is connected to a first flow route, which has flow channels, one of which is located radially between the pump wheel hub and the support shaft, the other radially between the support shaft and the gearbox input shaft. When the pressure in the hydrodynamic circuit is positive, the piston is pushed toward the adjacent housing cover of the clutch housing and is thus carried along rotationally by the clutch housing when the friction surfaces provided there engage. Conversely, this rotation in common stops when pressure medium is supplied through a second pressure medium line to a pressure space assigned to the piston and located axially between the piston and the housing cover. As a result of this supply of fluid, the pressure in this pressure space becomes greater than that in the hydrodynamic circuit, and the piston is thus shifted axially toward the hydrodynamic circuit. The second pressure medium line is connected to a second flow route, which leads through a central bore in the gearbox input shaft. Each of the two flow routes is connected to its own fluid reservoir.
The essential principle of this type of two-line system is that the bridging clutch serves as a separation point between the hydrodynamic circuit and the pressure space. This arrangement also represents the essential disadvantage the two-line system for the following reason. The friction surface, at least one of which is provided, fulfills two different functions when the bridging clutch is engaged, namely, that of transmitting torque from the clutch housing to the takeoff so that the hydrodynamic circuit is bypassed, and also that of sealing the hydrodynamic circuit off against the pressure space, which is essentially pressureless in this operating state. If the sealing action is inadequate because of overgenerous grooving in the area of the friction surfaces, for example, an undesirably large volume clutch fluid will be discharged from the hydrodynamic circuit via the bridging clutch into the pressure chamber and thus out of the clutch housing, and this lost volume would have to be replaced from the fluid reservoir. Conversely, if the sealing action in the area of the bridging clutch is too effective because the grooving in the area of the friction surfaces is very limited or because there is no grooving at all, the problem would result that the friction surfaces will become glazed very quickly and thus the friction linings in the area of the friction surfaces will be destroyed, especially when the slippage between the piston and the clutch housing causes the temperature of the minimum of one friction surface to increase. It is therefore necessary to produce a precisely defined flow in the area of the friction surfaces, and for this purpose it is necessary to conduct extensive testing with grooving of different dimensions. But even if the grooving is correctly dimensioned, wear or manufacturing tolerances will make it impossible to maintain the exact, desired volume flow rate of clutch fluid.
To solve this problem, designs are known in which the grooving no longer determines the volume flow rate passing through the bridging clutch in the engaged state. Instead, at least one point of throttled flow performs this function, which, according to U.S. Pat. No. 5,732,804, is provided in the piston of the bridging clutch in the radial area of the minimum of one friction surface. This throttled flow serves to supply a precisely defined volume flow rate of clutch fluid to the grooving assigned to the minimum of one friction surface. Although this eliminates the need to perform complicated tests to determine the dimensions of the grooving, it has been found that bridging clutches with these types of throttled flows can provide only certain areas of the friction surfaces with a flow of fluid and that it is impossible to prevent with sufficient reliability certain other areas from becoming overheated.